1. Field of the Invention
The present invention relates to a coating composition capable of forming a coated film having excellent properties in corrosion resistance, boiling water resistance and fabrication properties, and having excellent coating workability.
2. Description of the Background Art
A precoated steel sheet prepared by coil coating or the like has widely been used in the art as building materials such as roofs, walls, shutters and the like for an architectural structure, and as housing-related goods such as various kinds of household appliances, ventilation fans, panel boards, steel furniture and the like.
The above building materials and goods are prepared by a process which comprises cutting the precoated steel sheet and subjecting to fabrication such as press molding, and consequently have an edge face on which a metal surface has been exposed by cutting and a fabricated portion. A portion of the cut edge face may more easily be corroded compared with other uncut portions, and cracking on the fabricated portion may result in developing blisters and rust.
Coating of a coating composition containing a chromate-based anticorrosion pigment in a large amount has been carried out in the art for the purpose of improving corrosion resistance in the above cut edge face and fabricated portion with cracks. However, use of a large amount of the anticorrosion pigment in the coating composition may raise such problems that since the anticorrosion pigment generally has high solubility in hot water, dipping of a resulting coated film into a boiling water may result in developing blisters on the surface of the coated film.
For the purpose of reducing crackings developed in the above fabricated portion, use of a coating composition comprising a combination of a high molecular weight polyester resin or epoxy-modified polyester resin as a resin component of the coating composition with a curing agent has been proposed in the art. However, the use of the above coating composition raises problems of such a poor coating workability that viscosity control to be suitable for coating results in undesirably lowering a nonvolatile matter content because of high viscosity of the coating composition, that applicability in carrying out roll coater coating may be reduced, and that even if the applicability should be satisfactory, a high speed coating at a line speed of 80 m per minute or higher may result in forming a roll mark on a coated film, forming a non-uniform coated film, or developing film defects such as cissing.